Lab 7 Measurement of Ozone

Transcription

1 Georgia Institute of Technology School of Earth and Atmospheric Sciences EAS 4641 Spring 2007 Lab 7 Measurement of Ozone Purpose of Lab 7: In this lab you will measure the ambient concentration of ozone (O 3 ) for at least a 24-h period. The goal is to gain insight into methods for measuring trace gases and to investigate the sources of O 3. Background, Sources of O 3 Tropospheric ozone (O 3 ) is a secondary pollutant that is formed photo-chemically from chemical reactions involving oxides of nitrogen and hydrocarbons. In this process sunlight (hv) converts NO 2 to NO and atomic oxygen. The atomic oxygen then reacts with O 2 to produce O 3. NO 2 + hv NO +O O +O 2 +M O 3 + M Note that a major source for NO 2 is combustion (e.g., vehicles). These two reactions, however, do not lead to a net increase in O 3 since the O 3 formed can rapidly be converted back to NO 2 (see reaction below). Hence these first three reactions form a closed cycle. O 3 + NO NO 2 + O 2 Industrial processes, vehicles, and vegetation produce volatile organic compounds (VOCs) that can disrupt this closed loop and serve as the fuel for O 3 production. The VOCs, (which can be generically represented by RH), consume NO and produce more NO 2, which by the reactions above lead to formation of more O 3. RH + OH +O 2 RO 2 + H 2 O RO 2 + NO RO + NO 2 RO + O 2 R CHO + HO 2 HO 2 + NO OH + NO 2 The O 3 formed is referred to as photochemical smog since the key reactions, HO 2 conversion to NO, and the production of OH (mechanism is not shown), require energy from sunlight (hv). 1

2 EPA standards for Criteria Pollutant O 3 The Clean Air Act, last amended in 1990, requires the EPA to set National Ambient Air Quality Standards (NAAQS) for pollutants considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings. National Ambient Air Quality Standards have been set for six principal pollutants, called "criteria" pollutants. Units of measure for the standards are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m 3 ), and micrograms per cubic meter of air (µg/m 3 ). To protect and promote human health and public welfare the EPA attempts to: Mitigate potentially harmful human and ecosystem exposure to the six criteria pollutants: CO, NO 2, SO 2, O 3, PM(2.5, 10), and lead (Pb). The EPA must describe the characteristics and potential health and welfare effects of these pollutants. Limit the sources of and risks from exposure to hazardous air pollutants (air toxics). Protect and improve visibility impairment in wilderness areas and national parks. Reduce the emissions of species that cause acid rain, specifically SO 2 and NOx. Curb the use of chemicals that have the potential to deplete the stratospheric O 3 layer. O 3 NAAQS Effective January ppm maximum daily 1-hr avg to be exceeded no more than 1/yr averaged over 3 consecutive years 0.08 ppm 3-yr avg of the annual fourth highest daily 8-hr average See the EPA web site for more details on NAAQS: General Methods for Measuring Trace Gases Many techniques exist for measuring trace gases. Two methods for measuring O 3 are commonly used; both are based on spectroscopic techniques. One method is based on the property that O 3 molecules strongly absorb UV light of wavelength 245 nm. In the other method, O 3 is reacted with NO or ethane to produce an excited molecule that rapidly relaxes, emitting a photon of light that can be detected. Some details of the specific instrument used in this lab are presented next. Model 49C Ozone Analyzer O 3 is quantified in this instrument by measuring the absorption of UV light at wavelength 254 nm. The instrument is based on Beer s Law, which describes how a specific wavelength of light is absorbed by a particular gas molecule over a specified distance. where I = I 0 exp(-a L C) I: light intensity after absorption (ie, after traversing the absorption cell) 2

3 I 0 : intensity of the light entering the absorption cell a: absorption coefficient (how well the O 3 absorbs at a specific wavelength) L: absorption path length C: concentration of the absorbing gas (O 3 in this case). To determine C, the concentration of O 3 in Beer s Law, the instrument measures the ratio of I to Io. The other parameters in Beer s Law are known, L = 38 cm, and the molecular absorption coefficient, a = 308 cm 2 (at 0 C and 1 atmosphere). A schematic of the instrument is shown in the Figure below. Refer to the figure during the following description. To measure I and Io simultaneously, sample air is split into two streams. One gas stream flows through an O 3 scrubber to serve as the reference gas (the Io measurement) the other flow remains unchanged and is the sample gas (the I measurement). There are two identical detection cells (A and B) in the instrument. A solenoid valve alternates the reference and sample gas streams between cells A and B every 10 seconds so that any differences between the cells can be compensated. For example, the solenoid switches and reference gas flows through cell A and sample gas through cell B. The instrument than measures the absorption of 254 nm light in each cell, calculates the O 3 concentration for each cell, and outputs the average concentration (volts) to both the front panel display and the analog outputs, which are recorded by the computer. After 10 s the solenoid valves switch so that sample gas now flows through cell A and reference gas flows through cell B. The absorption measurement is made and O 3 concentration determined and displayed. The cycle is continued. A calibration must be performed to convert the voltage from the photodetector to O 3 concentration. Ozone Analyzer Schematic 3

4 Experiment No 7: O 3 in Urban Atlanta The following experiment will involve measuring the concentration of O 3 from the ES&T Penthouse Lab. To investigate the diurnal profile of O 3 concentrations, data will be collected continuously for ~24 hours. Met data will also be collected and used in the analysis. The first task will be to calibrate the O 3 monitor, but first make sure the clocks on the data acquisition computers are synchronized to within a few minutes. After calibrating, run the instruments for at least 24 hours. Download all data and perform the analysis discussed below. For comparison, see web sites for other O 3 and PM2.5 mass measurements in the state, including Atlanta, ( or Method Calibration In this lab an instrument is used to create a known concentration of O 3, similar to the HNO 3 perm tube in an earlier lab. We will assume that this PS (Primary Standard) Calibrator produces an accurate O 3 concentration. To calibrate the O 3 monitor, deliver O 3 concentrations of 180, 120, 80, 40, and 0 ppb from the O 3 PS Calibrator to the O 3 Analyzer (using the _ _ ENTER keys on the front of the PS Calibrator). For each O 3 concentration setting, let the instrument stabilize for ~ 3 minutes before making a measurement. Then, for roughly two minutes, record the O 3 output concentration from the Calibrator screen, (estimate the average so that you can compare with the computer data use the computer data in the analysis). The corresponding output voltage from the O 3 Analyzer will be saved to a file on the computer. Record the start and stop time for each 2-minute calibration. You will identify the voltages recorded by the computer by the time you were performing each experiment. This data will be used to construct the calibration curve to determine ambient O 3 concentrations. Ambient Measurement Disconnect the sample line delivering O 3 calibration gas from the O 3 PS Calibrator. Connect the ambient sample line to the sample inlet port on the O 3 Analyzer and record the time. Make sure the met station is operating and recording data during your measurements. Run the O 3 Analyzer Met Station for ~24 hours. When finished download the data and perform the analysis discussed below. 4

5 Discussion/Questions 1. Construct a calibration curve that to covert the Ozone Analyzer voltage to ozone concentration. This means plotting the calibration data and fitting via linear regression. (Use the data stored on the computer). 2. From the fit, determine the detection limit (DL) and accuracy (uncertainty) of the instrument. a. DL can be estimated two ways 1) from the uncertainty in the zero air measurement (e.g., the standard deviation of data when the calibration O 3 concentration was zero converted to O 3 concentration), 2) the uncertainty associated with the linear regression (ANOVA) intercept (use 1 standard deviation). Determine DL using both methods and compare. b. Estimate and state the uncertainty in the O 3 measurement based on the linear regression and any other uncertainties you deem important. 3. Check the quality of your data by plotting a continuous time series for the 24-h or longer measurements. Check for extraneous spikes in concentrations. If justified (state reason in lab report), remove spikes. 4. Compare your data to other sites in the city or surrounding region. Do you think the penthouse lab provides a site for a representative measurement of O 3 Atlanta, justify your answer. 5. Do you expect sampling losses of O 3 in the tube running from the inlet to the instrument? Briefly explain why. 6. Investigated the 24-hour variation of O 3. Also, you can plot diurnal variations in key meteorological data and compare with the O 3 variability. Explain the cause for any diurnal trends, or lack of trends, or any abrupt changes in concentrations. Does O 3 concentration peak, is the peak at the expected time. Support your arguments. 7. Only considering your measurements, would Atlanta be in compliance with the 24-hour air quality standards for O 3, for your sampling period? 5